Field
This disclosure relates to noise reduction headphones, and specifically to noise reduction headphones capable of high fidelity reproduction of an audio input.
Description of the Related Art
A speaker is a transducer for converting electrical signals into acoustic waves. Typical speakers include a diaphragm or other flexible element that moves in response to an audio input signal. Typical speaker diaphragms are flat or have a conical shape, in which case the diaphragm is typically referred to as the “speaker cone”. In either case, the motion of the diaphragm in response to the audio signal generates acoustic waves in the surrounding air. In a typical speaker, the diaphragm is mechanically coupled to a coil (commonly called a “voice coil” since early speakers were used to reproduce voice sounds) suspended in a magnetic field of a permanent magnet. An audio signal, in the form of a current passing through the coil, causes the coil to be alternately attracted and repelled by the magnetic field, resulting in corresponding motion of the diaphragm.
A headphone is a device that generates acoustic waves directly at a user's ear, and typically both ears. A typical headphone includes, for each ear, a housing and a flexible member intended to provide a seal between the housing and the user's ear or head. One or more speakers within the housing generate acoustic waves directed into the user's ear.
A noise reducing headphone is a type of headphone in which the speaker generates acoustic waves, commonly called “anti-noise”, intended to cancel, at least in part, ambient noise. To effectively cancel a sound, the anti-noise has to have the same amplitude and frequency spectrum as the sound entering the user's ear, with each frequency component of the anti-noise shifted in phase by 180 degrees with respect to the corresponding frequency component of the sound. Because canceling a sound is general impractical, the discussion herein is with respect to reduction, with a reduction to zero the same as cancellation.
Throughout this description, elements appearing in figures are assigned three-digit reference designators, where the most significant digit is the figure number where the element first appears, and the two least significant digits are specific to the element. An element that is not described in conjunction with a figure may be presumed to have the same characteristics and function as a previously-described element having the same reference designator.